The Emerald Ash Borer (EAB), Agrilus planipennis, is a highly destructive invasive beetle that has killed tens of millions of ash trees (Fraxinus species) across North America. This pest, originating in Asia, attacks both stressed and healthy ash trees, disrupting the tree’s ability to transport water and nutrients. Homeowners seeking sustainable management can use non-chemical strategies focused on boosting tree health, utilizing natural predators, and implementing physical removal techniques. These integrated methods offer a long-term approach to controlling EAB populations without relying on synthetic insecticides.
Identifying the Infestation and Tree Health
Accurately confirming an infestation is the first step in successful management. A clear diagnostic sign of EAB is the presence of small, distinct D-shaped exit holes, about 1/8 inch in diameter, left by emerging adult beetles on the bark. The most damaging activity occurs beneath the bark, where larvae feed on the inner wood layer. This feeding creates characteristic serpentine or S-shaped tunnels, known as galleries, which girdle the tree and cut off nutrient flow.
Infestation also manifests visually through the tree’s canopy, which begins to thin and die back, typically starting at the top. Another indicator of stress is the appearance of epicormic shoots, or water sprouts, growing rapidly from the trunk or main limbs. Assessing the tree’s health is necessary before intervention, as trees with more than 50% canopy dieback are unlikely to recover.
Strengthening Ash Tree Resistance
Good cultural practices are foundational to any non-chemical management plan, as maintaining optimal vigor helps the tree withstand minor infestations. This involves managing the tree’s environment to ensure consistent access to water and nutrients while minimizing physical harm. Proper watering is important, especially during drought, and should be deep and infrequent to encourage strong root development. The soil should be kept moist but not saturated or excessively dry.
Mulching helps regulate soil temperature and moisture. Apply a three to four-inch layer of organic mulch in a wide ring around the trunk, keeping the material several inches away from the bark itself. This prevents moisture accumulation against the trunk, which can lead to rot and disease.
Pruning should be kept to a minimum, focusing only on removing dead, damaged, or weak limbs, as excessive pruning induces stress. Removing low-vigor branches can help slow the buildup of EAB populations. Limiting pruning wounds is beneficial, as injuries to the bark create potential entry points for pests and pathogens.
Employing Biological and Physical Controls
Biological controls offer a long-term, landscape-level solution by introducing natural enemies to suppress the beetle population. The United States Department of Agriculture (USDA) has approved the release of several species of tiny, stingless parasitoid wasps that are specialized predators of the EAB. These gnat-sized wasps pose no threat to people or pets.
These biocontrol agents work slowly over many years to establish a self-sustaining population that continually reduces the number of beetles in an area. Native predators, such as woodpeckers, also play a significant role in control by feeding on EAB larvae beneath the bark.
Parasitoid Wasps
The approved parasitoid wasps target different life stages of the EAB:
- Tetrastichus planipennisi targets EAB larvae developing under the bark.
- Oobius agrili is an egg parasitoid that lays its own eggs inside EAB eggs.
Physical controls involve direct action to monitor and eliminate the pest and its habitat. Sticky trunk traps are a common physical tool used for monitoring the presence and population levels of adult EAB. These traps are diagnostic in nature, providing data on beetle activity, but they are not an effective method for eradicating an infestation on their own.
Mechanical removal of infested wood is necessary when a tree’s canopy decline exceeds 50%. If an ash tree must be removed, the infested wood requires careful handling to prevent the spread of the beetle to nearby trees and communities. Proper disposal involves chipping or burning the wood, if local regulations permit, or heat-treating it at 140°F for 60 minutes to kill any remaining larvae. Homeowners must also avoid moving ash firewood outside of local areas to prevent further infestation.